Inverters are known generally and are devices which transform DC (direct current) electrical energy, such as from a fuel cell or the like, into AC (alternating current) electrical energy suitable for use by utility companies or other consumers of electrical energy. Most inverters include at least one pair of main switching elements, known as thyristors, and by sequentially actuating each switching element, each DC bus is sequentially connected to the load so that electrical energy flows first in one direction and then in the reverse direction forming a fundamental AC waveform.
Numerous different types of switching devices can be employed in an inverter as a switching element to connect the DC buses to the load. One well known type, called a thyristor, is often used because of its ability to handle large currents. Although thyristors change from the nonconducting state to the conducting state in response to a suitable control signal applied to their control terminal, the reverse process, changing from the conducting state to the nonconducting state, is much more complicated. To switch to the nonconducting state, most thyristors require that the magnitude of the instantaneous anode-to-cathode current be reduced to zero, a reverse voltage be applied to the thyristor and the control signal removed from the control terminal in order to allow the thyristor to transition to its off state.
The term "commutation" has become known as the process by which a thyristor is transitioned from its conducting state to its nonconducting state and numerous circuit configurations are known for this function. Most commutation circuits operate by presenting a commutation pulse to the load from a storage device, such as a capacitor or resonant circuit, that displaces the current flowing through the thyristor and presents a reverse voltage to the thyristor. If the commutation pulse applies a reverse voltage for a period of time which exceeds the "turn-off time" of the thyristor, and the signal from the control terminal is removed, the thyristor will transition to its nonconducting state.
There is a continuing interest in improving the efficiency of the conversion of DC electrical energy to AC electrical energy by the inverter. Of interest in this area is U.S. Pat. No. 4,204,268 issued May 20, 1980 to J. Vivirito for AUXILIARY COMMUTATION CIRCUIT FOR AN INVERTER, and assigned to the same assignee as the present invention. This patent discloses an auxiliary commutation circuit of the impulse commutated bridge inverter type in which additional commutation energy is stored in a pair of oppositely charged capacitors. Switched elements in series with the capacitors are operable in response to sensed overcurrent conditions to provide additional stored energy during commutation.
Another disclosure related to the auxiliary commutation concept is U.S. Pat. No. 4,225,912 issued Sept. 30, 1980 to G. Messer for CONTROL FOR AN AUXILIARY COMMUTATION CIRCUIT, also assigned to the same assignee as the present invention. The auxiliary commutation circuit in this disclosure is actuated only during overload conditions which improves overall inverter efficiency. A control circuit responds to the increase time period of the commutation pulse to delay the firing of the thyristors which initiate the makeup pulse. This modified operation continues for at least three commutation cycles in order to ensure that the supplemental portion of the commutation circuit is properly initialized so that it is ready for subsequent overcurrent conditions.
Another efficiency improving technique is disclosed in U.S. Application Ser. No. 43,195, filed May 30, 1979 by G. Messer for SELECTIVE COMMUTATION CIRCUIT FOR AN INVERTER, and also assigned to the same assignee as the present invention. This commutation technique senses when the main thyristors cannot be transitioned to the nonconducting state by the removal of a control signal to the gate terminal of the main thyristors. If a commutation pulse from the commutation circuit is required, the commutation capacitors are charged to the appropriate voltage level immediately prior to the commutation point so that there is a sufficient pulse to extinguish the load current through the thyristor. After commutation, the commutation capacitors are returned to a stable voltage level until another commutation pulse is required.